Gear pumps are used for conveyance of highly viscous fluid, such as polymer melts. For example, gear pumps are typically used for conveying a viscous polymer melt from a vessel, such as a devolatilizer, to another unit operation, such as a pelletizer. In most cases, the highly viscous polymer melt enters the pump inlet under the influence of gravity with essentially no positive pressure. Known gear pumps are susceptible to a number of difficulties in their operation. In particular, for any given pump geometry, known gear pumps are extremely limited with respect to the range of viscosity of fluids that they can handle. Generally, as fluid viscosity increases, the throughput rate of the gear pump decreases, often resulting in a production bottleneck. Also, in general, as gear pump speed (RPM) increases, pump throughput initially increases, but eventually reaches a plateau level, wherein further increases in pump speed do not result in any significant increase in throughput and can lead to a production bottleneck. Heretofore it has generally not been possible to effectively overcome a production bottleneck of this type once the plateau level of the pump speed verses pump throughput has been reached without replacing the existing pump with a larger pump. However, the devolatilizer is typically specially configured to be coupled to a gear pump of a particular size, and it is not generally possible to switch to a larger capacity gear pump of conventional design without also replacing or significantly modifying the devolatilizer. Accordingly, it would be highly desirable to provide a gear pump which operates more efficiently to eliminate such production bottlenecks without requiring replacement or significant modification of the devolatilizer.
Various attempts have been made to design gear pumps which are capable of operating efficiently over a wider range of fluid viscosity and over a wider range of pump speeds. These efforts have focused primarily on pump geometry, particularly at the inlet side of the pump. However, the known pump designs have not been entirely satisfactory and further improvements are desirable.